This invention relates to a data transmission apparatus and a data transmission system, and more particularly to a data transmission apparatus suitable for use for transmission of audio visual data for which a protocol of the IEEE (Institute of Electrical and Electronics Engineers) 1394 High Performance Serial Bus Standards (hereinafter referred to as IEEE 1394 serial bus) is used.
It is highly anticipated that the IEEE 1394 serial bus will be spread widely in the future as a digital interface for connection between a world of digital audio visual apparatuses and a world of a personal computers. The reason why such high anticipation is directed to the IEEE 1394 serial bus is that it has such significant characteristics as given below. In particular, while a related art audio visual apparatus (hereafter referred to as AV apparatus) normally employs a DS link wherein data transmission is performed by two-value difference driving, the IEEE 1394 serial bus realizes three-value difference driving at a high data transfer rate of 100, 200, 400 Mbps by simultaneous driving of two nodes linked to each other by a shielded 6-pin cable including a twisted line and a power supply line in a pair.
Where the IEEE 1394 bus is used to communicate audio digital data (hereinafter referred to as AV data) between two arbitrary nodes, communication data are divided into packets and transmitted in time division in cycles of the interval of 125 .mu.s. More particularly, isochronous communication in which an isochronous packet is used and asynchronous communication in which an asynchronous packet is used are performed suitably. AV data are transmitted basically with isochronous packets. For example, two nodes which intend to communicate AV data using isochronous communication access, in order to secure a data transmission band for the isochronous communication when they act as a transmitter and a receiver, utilize a predetermined address of a node (hereinafter referred to as route), which serves also as a management apparatus of a transmission line, by asynchronous communication in a time band which remains in each cycle and is not used for data transmission of isochronous communication to request for a bandwidth (channel) to be used for the isochronous communication.
In response to the request, the route performs arbitration (allocation of a bandwidth) and provides an isochronous reference (hereinafter referred to as IR) issued from an isochronous resource manager (hereinafter referred to as IRM) built therein to each of the nodes successively beginning with the node to which the route has assured a bandwidth for communication. Each of the nodes on the IEEE 1394 serial bus performs isochronous communication with the IR supplied thereto and consequently can transmit or receive AV data connected thereto to or from the other node. After the communication of AV data is completed, each of the nodes returns its IR to the IRM of the route.
According to the transmission system described above, however, also when a codec has a variable compression ratio of transmission data such as a wavelet transform codec used to transmit AV data in accordance with an image compression/decompression protocol having a variable transmission bandwidth, AV data are transmitted by isochronous communication corresponding to the fixed used bandwidth determined by the IRM upon establishment of connection between the nodes. Consequently, the related art transmission system described above has a subject to be solved in that, even when the occupied bandwidth of the bus varies during the communication of AV data and a greater margin appears with the bandwidth of the transmission line or the margin of the bandwidth of the transmission line disappears, it is difficult to suitably vary the transmission bit rate of transmission data in response to the variation of the occupied bandwidth of the bus.